This invention relates to rear projection screens, and more particularly relates to such screens having a vertically oriented, parallel array of steep-sided lenticular elements on the front surface, for spreading light horizontally into the audience space.
Such a screen is known from my U.S. Pat. No. 4,573,764, in which a remarkably wide horizontal angle of view of greater than plus or minus 85 degrees relative to the projection axis is accomplished by combining a conventional light collimating circular Fresnel lens with a light distributing lenticular lens array in which the individual lenticular elements have critical design parameters including height-to-width ratio and overall width-to-tip width ratio, as well as tip regions characterized by two lateral convex and central concave cylindrical elements, resulting in exceptional horizontal dispersion of light, while at the same time exhibiting minimal color shift.
The design of the lenticular elements results in steep sides on which some light is reflected by the phenomenon known as total internal reflection (TIR). This tends to concentrate the light in the tip region of the lenticular elements before it passes into the audience space. For this reason, the front lenticular lens array is referred to herein as the TIR array.
Dispersion of the light rays in the vertical direction is generally accomplished by diffusion means located between the light collimating Fresnel lens and the light spreading lenticular lens array.
In the case of projection television, it is usually not necessary to distribute the light through a large vertical included angle, since the individual vertical viewing angles within the audience typically vary only a few degrees from one another. On the other hand, for optimal performance of the TIR array, it is advantageous to spread the light horizontally before it reaches the TIR array. Unfortunately, the amount of diffusion needed to achieve adequate horizontal spreading can significantly reduce screen gain. Furthermore, the spreading of light due to diffusion is not optimal for the performance of the TIR array with diffusion, some rays are deviated through too great an angle to usefully illuminate the TIR array, so loss performance, e.g., increased color shift, can occur. A vertical lenticular array with a small pitch placed behind the TIR array can be used to effect the desired horizontal spreading of the light, but can result in objectionable moire.
The use of crossed lenticular arrays to control spreading of light in two different directions is known from U.S. Pat. Nos. 2,618,198 and 3,830,556. However, these arrays were the only means used to control the light entering the screen and the audience space. If such arrays were combined with other optical elements or surfaces such as a circular Fresnel lens and/or a TIR array, (see, for example, U.S. Pat. No. 4,432,010) screen gain would be compromised due to transmission and/or reflection losses, the exact nature of which would be determined by the particular screen structure and materials chosen.